Liver failure is the seventh leading cause of death in the USA, with 300,000 hospital admissions per year and more than 40,000 deaths. Liver failures are classified as acute or chronic. A common type of acute liver failure is Fulminant Liver Failure (FLF), defined as severe impairment of hepatic functions in the absence of pre-existing liver disease. The common FLF causes include viral hepatitis, poisoning, and stroke. Common types of chronic liver failure include viral or alcohol cirrhosis, drug and toxin induced diseases.
Spontaneous self-regeneration of liver occurs in about 25% of FLF patients (which is not always possible to predict) and in a very small percentage of chronic liver patients. The most common and often the only treatment option for liver failure is a liver transplant, at a typical cost of about $400,000 per transplant in the USA, making it a $2 billion industry in the USA alone. About 5,000 transplants are available in the USA per year, but there are 17,000 potential recipients on the waiting list. The organ shortage is expected to increase as the incidence of viral hepatitis type C (HVC) is continues to increase. One of the driving forces are the 4 million Americans currently infected with HVC, 15% of these are expected to require a transplant. The total number of chronic liver failures is projected to triple between 2002 and 2010.
There has been intensive research into cell-containing liver-assist devices (LADs) for over 20 years and several devices have achieved clinical trials. In FLF treatment, LADs promise the patients a temporary relief of liver metabolic load and a chance for the liver to regenerate itself. In this case, LADs prevent wastage of a donor organ. In chronic liver failure treatment, LADs promise a bridge to transplant, and, potentially a long term support option.
However, none of the cell-containing LADs has successfully passed all clinical trials and achieved an FDA approval. The main problem is the poor long-term device performance due to a low viability and a loss of differentiated function of hepatocytes. A second problem is the lack of biliary excretion. A third problem is a limited device performance, even when the hepatocytes are still functional, due to non-uniform cell distribution and non-uniform flow distribution. A fourth problem is a low cell density, resulting in a need for a relatively large device size. Advantages and drawbacks of prior art are detailed in the following paragraphs.
The hollow fiber design was the earliest design concept and has advanced farthest through clinical trials. The cells are attached inside the fibers, possibly trapped in a gel, or outside the fibers. The blood or other nutrient-containing medium is perfused to the opposite side of the fiber. The assembly and manifolding of several thousand of these hollow fibers into a bioreactor is a complex and slow process. Cells are perfused after the device assembly. Cell density has been relatively low. Additionally, attempts to culture different cell types in the fibers have proven unsuccessful. Because of low cell density and the difficulty of assembling thousands of hollow fibers, long fibers tend to be used, resulting in non-uniform conditions along the fiber length. An attempt at supplying different medias through several sets of hollow fibers resulted in very complex geometry, and limited scale-up to about 2.5% of physiological human liver cell numbers.
The perfused bed scaffolds typically comprise three dimensional open pore architectures. These scaffolds are easy to scale up, can be shaped with rapid prototyping tools with a narrow pore distribution in a target range. Pore sizes can be adjusted to promote high cell density and vascularization. However, in LAD applications, these scaffold designs have several drawbacks, including residual chemicals, no provision for separation of metabolic flows, and no provision for patterning different cell types in target geometries, non-uniform perfusion, clogging, and exposure to shear.
The encapsulated cell suspension design offers the ease of scale-up and a uniform microenvironment. The drawbacks include poor cell stability and loss of differentiated function (particularly, for hepatocytes, which are attachment-dependent cells), barriers to nutrient transport due to encapsulation, degradation of capsules over time and exposure of cells to shear forces.
The flat plate design is another known device. In this method, it is possible to achieve uniform cell distribution and microenvironment. Multiple patents have recognized the advantage of multilayer arrangements, where cells can be sandwiched between collagen or biologically derived matrices (basal lamina, Matrigel) and oxygen permeable membranes, such as U.S. Pat. Nos. 3,734,851 and 5,605,835. Several groups have verified modulation of hepatocyte polarity and formation of canaliculi. U.S. Pat. No. 5,605,835 shows an example of multilayer arrangement, stacked up to 50 layers and up to 200 layers and individually piped. In order to provide mechanical strength during assembly, the individual plates have to be relatively thick. The disadvantages of this design include exposure of cells to shear, low surface to volume ratio, low cell density, and complex scale-up.
A further improvement to flat bed design came from co-culturing layers of stromal cells with hepatocytes, which was demonstrated to increase viability and differentiated function of hepatocytes (U.S. Pat. Nos. 4,963,489; 5,510,254; 5,863,551; 6,008,049; 6,140,039; and 6,218,182). Further improvement came from separation of cells from oxygen containing medium by a membrane (U.S. patent application Ser. No. 200300017142), which allowed increased oxygenation without exposing cells to damaging flow shear stresses.
In order to create multilayer structures, it has been suggested to roll the substrate into a cylinder. In U.S. Pat. No. 6,218,182 two media, with different concentrations of solutes, flow on the ID and OD of the rolled-up cylinder. The purpose of such assembly is to create a diffusion gradient in the radial direction, which is believed to be beneficial for cell polarization and growth. The substrate may contain layers of multiple materials and different cell types. The products have to diffuse through multiple layers to reach and be removed by one of the media. The overall setup is geared to tissue growth rather than tissue function. The diffusion pathways are long (from ID to OD) and there is no provision for patterns within the layers or separation of various metabolic products.
In U.S. Pat. No. 6,372,495, hollow fibers are randomly embedded or attached to a porous sheet or mat. The fibers are minimum 100 microns OD and are arranged in parallel. The mat is 50 to 2000 microns thick (typically, about 500 microns) by 10-100 cm wide, and has 10-100 micron pores. The fibers are intended to serve as spacers during rolling-up the sheets, provide solid support for the sheets and exchange oxygen and carbon dioxide. After the roll-up, individual fibers (typically, 500 to 5000) are manifolded together and connected to a gas source, while space between the fibers is connected to liquid flow. After assembly, the porous media are infused with cells. Cells are protected from shear stresses by being embedded into the porous sheet. The invention has several drawbacks as discussed earlier: it is difficult to manifold several thousands of fibers, different cell types are not geometrically organized, and the diffusion path is longer than in a live liver. Also, metabolic products of the cells stay in the circulating liquid, and may, eventually, negatively affect the cell viability and differentiation.
Hence it remains desirable to provide a biomimetic liver that avoids the drawbacks of the prior art.